A phosphor is utilized in a fluorescent display tube (VFD: vacuum-fluorescent display), a field emission display (FED: Field Emission Display) or SED (Surface-Conduction Electron-Emitter Display), a plasma display panel (PDP: Plasma Display Panel), a cathode-ray tube (CRT: Cathode-Ray Tube), a liquid display backlight (Liquid-Crystal Display Backlight), a white light-emitting diode (LED: Light-Emitting Diode), and so on. In any of these applications, it is necessary to provide the phosphor with energy to excite the phosphor in order to make the phosphor emit fluorescence and the phosphor is excited by an excitation source with high energy such as a vacuum ultraviolet ray, an ultraviolet ray, an electron beam, and blue light so as to emit a visible light ray such as blue light, green light, yellow light, orange light, and red light. However, as a result of the phosphor being exposed to such excitation source, the luminance of the phosphor tends to decrease and a phosphor having little degradation in the brightness is desired. Therefore, a phosphor having an inorganic crystal containing nitrogen in a crystal structure thereof as a host crystal, instead a phosphor such as a silicate phosphor, a phosphate phosphor, a aluminate phosphor, and a sulfide phosphor, has been proposed, as exemplified by a sialon phosphor, an oxynitride phosphor, or a nitride phosphor, which is characterized by low brightness deterioration caused by high energy excitation.
An example of the sialon phosphors is manufactured by a manufacturing process as generally described below. First, silicon nitride (Si3N4), aluminum nitride (AlN), and europium oxide (Eu2O3) are mixed in predetermined molar ratios and the resultant mixture is fired by a hot press method in one atmospheric pressure (0.1 MPa) of nitrogen atmosphere at 1700° C. for one hour. It was reported that α-sialon activated with an Eu2+ ion manufactured by the above process had become a phosphor emitting light of a yellow color in a wavelength range of 550 nm to 600 nm if excited by blue light having a wavelength range of 450 to 500 nm. Further, it is known that an emission wavelength may vary as a ratio of Si to Al or a ratio of oxygen to nitrogen is changed while the α-sialon crystal structure is maintained.
As another example of the sialon phosphor, a green phosphor in which β type sialon (β-sialon) is activated by Eu2+ is known (For example, refer to the specification of Japanese Patent No. 3921545. The document is incorporated herein by reference.). It is known that, in the phosphor, an emission wavelength thereof varies to a shorter wavelength by changing the oxygen content while the crystal structure remains the same. Moreover, it is known that a blue phosphor is to be formed when β-sialon is activated by Ce3+ (For example, refer to WO 2006/101096 A. The document is incorporated herein by reference.).
As an example of an oxynitride phosphor, a blue phosphor having a JEM phase (LaAl(Si6−zAlz)N10−zOz) as a host crystal which is activated by Ce (For example, refer to WO 2005/019376 A. The document is incorporated herein by reference.) is known. It is known that, in the phosphor, an emission wavelength shifts to a longer wavelength as an excitation wavelength shifts to a longer wavelength by substituting partially La with Ca while the crystal structure is maintained.
As another example of the oxynitride phosphor, a blue phosphor having a La—N crystal La3Si8N11O4 as a host crystal, which is activated by Ce, is known (For example, refer to Japanese patent application publication No. 2005-112922 A. The document is incorporated herein by reference.).
As an example of a nitride phosphor, a red phosphor having CaAlSiN3 as a host crystal, which is activated by Eu2+ is known (For example, refer to Japanese patent No. 3837588 B. The document is incorporated herein by reference.). Color rendering properties of a white LED are improved effectively by utilizing this phosphor. It has been reported that a phosphor to which Ce is added as an optical activating-element becomes an orange phosphor.
Thus, an emission color of the phosphor is determined by a combination of the crystal to act as the host crystal and a metal ion (activating ion) being incorporated into the crystal. Further, the combination of the host crystal and the activating ion determines emission characteristics such as an emission spectrum and an excitation spectrum, chemical stability, and thermal stability such that a phosphor is regarded as another different phosphor when a host crystal thereof or an activating ion thereof is different. Moreover, a material having a different crystal structure is different in the emission characteristics or in the stability because the host crystal is different even if the material has the same chemical composition such that the material is regarded as another different phosphor.
Further, kinds of constituent elements can be substituted in many phosphors while the same crystal structure of the host crystal is maintained, thereby changing the emission color. For example, although a phosphor having a YAG crystal to which Ce is added emits light of a green color, a phosphor having a YAG crystal in which Y is partially substituted with Gd and Al is partially substituted with Ga exhibits emission of a yellow color. Further, in a phosphor having CaAlSiN3 to which Eu is added, it is known that a composition thereof varies by partially substituting Ca with Sr while the same crystal structure is maintained such that the emission wavelength shifts to a shorter wavelength. In this way, such a phosphor in which element substitution is performed while the same crystal structure is maintained is regarded as a material of the same group.